Hydrogen is one of the few molecules that has been incarcerated in the molecular cage of C60to form the endohedral supramolecular complex H2@C60. In this confinement, hydrogen acquires new properties. Its translation motion, within the C60 cavity, becomes quantized, is correlated with its rotation and breaks inversion symmetry that induces infrared (IR) activity of H2. We apply IR spectroscopy to study the dynamics of hydrogen isotopologues H2, D2 and HD incarcerated in C60. The translation and rotation modes appear as side bands to the hydrogen vibration mode in the mid-IR part of the absorption spectrum. Because of the large mass difference of hydrogen and C60 and the high symmetry of C60 the problem is almost identical to a vibrating rotor moving in a threedimensional spherical potential. We derive potential, rotation, vibration and dipole moment parameters from the analysis of the IR absorption spectra. Our results were used to derive the parameters of a pairwise additive five-dimensional potential energy surface for H 2@C60. The same parameters were used to predict H 2 energies inside C70. We compare the predicted energies and the low-temperature IR absorption spectra of H2@C70. © 2013 The Author(s) Published by the Royal Society. All rights reserved.
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